The present invention relates generally to a heat-sensitive recording method, and more particularly to a heat-sensitive thermal recording method applicable to a facsimile apparatus, in which excessive dot density variations appearing when a conventional method is performed can be reduced.
When high speed printing is performed by means of a conventional thermal recording apparatus, the printing dot density is unfavorably varied depending on the time intervals at which image data is printed on record paper. For example, in a case in which a heat-generating element of the thermal recording apparatus, or a so-called thermal head, is thermally driven by recording power pulses at time intervals of 1.8 msec, an image is formed with dots printed on record paper at a prescribed printing dot density. The operating condition of the thermal head in this case is periodically returned from a heated condition to a base temperature level every 10 msec. The printing dot density is determined primarily depending on the thermal energy applied from the thermal head to a color agent or image transfer agent with which an image is thermally recorded on record paper. The greater the thermal energy being applied by the thermal head in a heated condition is, the higher the printing dot density is. Accordingly, the conventional printing apparatus performs a thermal printing at subsequent timings while the operating condition is periodically returned to a base temperature level.
However, in a case in which the above mentioned recording method is performed, it is difficult to keep up with a high speed printing at a printing time period of 2.0 msec or below. When the thermal head performs a printing of subsequent dots on record paper, it still remains in a heated condition and is not returned to a base temperature level.
Some improved thermal recording methods have been proposed in order to eliminate the above mentioned problem. For example, Japanese Laid-Open Patent Application No.55-142675 discloses a heat-sensitive recording device which performs such an improved thermal recording method. In this conventional device, characteristics data of its thermal head defining a relationship between the printing time period and the recording power pulse duration or pulse width is previously stored, and an intended printing dot density is achieved by the printing time period thus stored. The actual printing time period of the thermal head is measured, and the widths of recording power pulses by which the thermal head is thermally driven are selectively determined on the basis of the characteristics data using the measured printing time period. Thus, the thermal head of the conventional apparatus can start printing at the intended printing dot density in its heated condition. Also, the conventional apparatus is operable in a case in which a printing time period intended for the printing is variable. Other improved thermal recording methods have been proposed, and the above mentioned problem is eliminated in those improved methods, for example, by changing the operating condition of the thermal head rapidly from a heated condition to a base temperature level, or by selecting a pulse width in response to the thermal head temperature.
A description will now be given of a relationship between the thermal head temperature and the recording power pulse width in a heat-sensitive recording apparatus. Generally speaking, it is desired that a heat-sensitive recording apparatus can provide an appropriate printing dot density in performing a heat-sensitive recording and no excessive printing density variations are produced. One conceivable method for reducing such undesired printing density variations is that the recording power pulse width Hpw is varied suitably with respect to the thermal head temperature T.
FIG. 2A shows an ideal characteristics chart which indicates a relationship between the thermal head temperature T and the recording power pulse width Hpw. As shown in FIG. 2A, this relationship is represented by two straight lines with different inclinations, the straight lines having an intersecting point appropriately at 20 deg C. In many cases, the characteristics chart indicating the relationship between the temperature T and the pulse width Hpw can be approximated by two such straight lines. An ideal method for eliminating the above mentioned problem is to select accurately a recording power pulse width Hpw on the basis of the characteristics chart as shown in FIG. 2A from the respective thermal head temperatures. However, in practical cases, only a few values of the recording power pulse width Hpw are predetermined for the respective temperatures T in the applicable temperature range (the range between 5 deg C and 60 deg C, for example), and a staircase-like chart formed with vertical and horizontal straight lines shown in FIG. 2B which can be approximate to the ideal characteristics chart shown in FIG. 2A is applied.
FIG. 3 shows a heat-generating portion of a heat-sensitive recording system. In FIG. 3, an 8-bit counter 31, a Schmitt input 32 and an open drain output 33 are provided for measuring a resistance value of a thermistor 34. The thermistor 34 is usually provided within a thermal head as the heat-generating element, and a resistor 35 with a resistance Ro is connected in parallel to the thermistor 34. The thermal head temperature T is determined from a resistance value of the thermistor which is measured with the 8-bit counter 31, and, from the thermal head temperature T thus determined, the recording power pulse width Hpw is selected o the basis of the characteristics chart. In general, the thermistor resistance value Th which can be measured with the 8-bit counter 31 is represented by the following formula: EQU Th.sub.n =Ro exp B (1/Tn-1/To )
In this formula, Th.sub.n is a thermistor resistance value at a thermal head temperature T.sub.n deg C, Ro is the thermistor resistance value at To deg C, B is the thermal sensitivity coefficient, and To is the reference temperature which is, for example, 25 deg C (298.15 K). As is apparent from this formula, the relationship between the thermistor resistance Th and the thermal head temperature T can be shown as a hyperbolic chart.
FIGS. 4A and 4B are hyperbolic charts each indicating the relationship between the thermistor resistance Th.sub.n and the thermal head temperature T.sub.n. FIG. 4A shows a case in which the thermistor resistance values are separated into constant steps Th.sub.n -Th.sub.n-1 =h:constant ). As is apparent from FIG. 4A, steps of the thermal head temperatures T corresponding to the thermistor resistance values Th with constant steps h become greater as the thermal head temperature T becomes higher ( T1&lt;T2&lt;T3&lt; . . . &lt;T.sub.n). Therefore, the relationship between the pulse width Hpw and the temperature T, as shown in FIG. 5, can be explained as follows: in a lower temperature range the pulse width Hpw can be approximate to the ideal case, but, in higher temperatures, steps of the pulse width Hpw become 1 excessively great, which will produce excessive printing dot density variations in a printed image.
On the other hand, FIG. 4B shows a case in which the thermal head temperatures are separated into constant steps (T.sub.1 -T.sub.n-1 =t:constant). In this case, as shown in FIG. 4B, the changes of the thermistor resistance Th corresponding to the thermal head temperature T with constant steps t become smaller and smaller as the temperature T becomes higher (Th.sub.1 &gt;Th.sub.2 &gt;Th.sub.3 &gt; . . . &gt;Th.sub.n).
Therefore, in practical cases, the changes in the actually measured thermistor resistance Th cannot keep up with the changes in the intended recording power pulse width Hpw, and, consequently, the steps of the pulse width Hpw in a higher temperature range will not become constant, while in a lower temperature range the steps of the pulse width Hpw will be varied very coarsely. Thus, in the case in which the conventional thermal recording apparatus is used, there is a problem in that excessive variations in printing dot density are produced, because the steps of the thermistor resistance value Th actually measured cannot keep up with the steps of the intended recording power pulse width Hpw.